This invention relates to powdery coating compositions suitable for high temperature coating of metallic surfaces comprising a saturated, high-molecular weight thermoplastic polyester binder and the customary auxiliary agents and additives, characterized by the addition of an improved flow agent.
Such coating compositions are utilized for the production of enamel-like coatings on metals in fluidized-bed coating, flame-spraying or electrostatic coating methods. The coatings are distinguished by high adhesion, good thermal and solvent stability, as well as excellent mechanical and electrical properties.
In practice, it has been found that such powdery coating compositions have several essential disadvantages in addition to their good properties. One of these disadvantages resides in poor flow characteristics during the coating process on a heated metallic substrate article, due to the relatively high melt viscosity of the polyester binders. Due to this fact, undulating surface coats are obtained; such an appearance is called the "orange-peel" effect.
This disadvantage can be avoided by adding to the coating composition flow agents which reduce the melt viscosity. Thus, in U.S. Pat. No. 3,654,198, cellulose esters and organic plasticizers are employed in combination to improve the flow properties. However, in order to obtain a sufficient effect, it is generally necessary to use relatively large amounts of the flow agent so that a marked impairment of the desirable properties necessary in enamel technology is encountered, e.g. an impairment of the elasticity. Besides, these additives have the disadvantage that they are decomposed and/or volatilized at the high temperatures employed in the coating step; this leads to the troublesome smoking phenomenon while the coating is being applied. The latter disadvantage especially must be avoided in view of the environmental protection regulations nowadays in existence.
Also, other high-molecular weight additives are known by means of which the flow of coating compositions which contain powdery, high-molecular weight polyester binders can be improved. For example, a technical leaflet from Dynamit Nobel A. G. describes acrylate resins as flow agents for the P 1300 type polyester. However, these products likewise tend toward smoking at the high processing temperatures employed in the production of enamel-like coatings.